This invention relates to a magnetic field sensor including at least one Hall element that is connected to the input of a differential single pole high pass filter which may be further connected to the input of a comparator or Schmitt trigger circuit for counting the passage of ferromagnetic articles.
There is disclosed in the U.S. Pat. No. 4,218,659, issued Aug. 19, 1980, several AC coupled Hall voltage amplifiers connected to the output of a Hall element for use in a magnetic pick-up or play back head intended for detecting signals on a passing magnetic tape or the like. These amplifiers employ five or six capacitors and provide high pass filtering with numerous poles. These circuits are aimed at reducing the thermal noise of the Hall element, circuit resistors and transistors of the sensor head itself that tends to be combined with the sensing head output signal.
It is an object of the present invention to provide a ferromagnetic article sensor having a Hall element and a Hall signal high pass filter with only one filter capacitor so that the output signal is substantially free of electrical impulses and other noise that may be present on the DC power lines as well as the DC voltage component of the Hall output voltage.